Spanning an area of about 3,861,400 square kilometres (1,490,900 sq mi) and with a mean discharge of about 30,900 m3/s (1,090,000 cu ft/s),[2] the basin is almost entirely within Canada.

A small area of the basin is in the northern part of the Midwestern United States.

[3] The Hudson Bay's connection to the Labrador Sea (the Atlantic Ocean) is at the Hudson Strait's mouth, between Resolution Island in the Qikiqtaaluk Region and Cape Chidley on the Labrador Peninsula.

Other authorities include it with the Atlantic Ocean, in part because of its greater water budget connection.

While the Nelson runs a relatively short 644 kilometres (400 mi) from Lake Winnipeg to the Hudson Bay, its tributaries extent as far west as the Rocky Mountains.